This invention relates to a process for preparing polyurethane and/or polyurea elastomers (PUU elastomers), and particularly to a process employing a relatively high equivalent weight polymer of propylene oxide.
Polyurethane and/or polyurea elastomers are commonly prepared by reacting a polyisocyanate with a relatively high equivalent weight "polyol" and a chain extender in a mold. The "polyol" component is typically a material having an equivalent weight in the range from about 800 to about 5,000 which contains 2 or more isocyanate-reactive groups per molecule. One common type of these is a polymer of propylene oxide. These poly(propylene oxide) polymers have found extensive use in preparing various types of polyurethane or polyurea elastomers. Polymers of propylene oxide provide the elastomer with very desirable properties, including flexibility, low temperature performance, toughness and softness. They are also relatively inexpensive and can be prepared having varying functionalities. Accordingly, they are the materials of choice in many elastomer applications.
However, in some instances, it is more difficult than desired to process these polymers into polyurethanes. In order to maximize production from an individual mold, the mold must be used and reused as often as possible per unit of time. Thus, it is desired that the PUU-forming composition react quickly inside the mold so that the elastomer rapidly gains enough physical strength that it can be demolded without becoming permanently distorted.
Previously, there have been attempts to reduce demold times by increasing the reactivity of the reaction mixture. For example, the "polyol" can often be made more reactive by providing terminal primary hydroxyl groups. This is most commonly done by "capping" it with ethylene oxide. However, since not all chain ends are capped, it is usually necessary to use a substantial quantity of ethylene oxide to obtain a high conversion to primary hydroxyl groups. The capped polymer therefore contains a significant proportion of oxyethylene groups, which cause it and the resulting PUU elastomer to be moisture sensitive. In addition, this approach by itself often cannot increase the reactivity of the polyol as much as desired.
Another approach is to increase catalyst level. Although reduced demold times can be obtained in this manner, premature gelation of the reaction mixture and other unwanted side effects often occur. Because of these, the molder often has an extremely short time to transfer the reaction mixture into the mold, which limits the size of the part he can make. These problems are particularly severe in reaction injection molding (RIM) processes, wherein especially highly reactive components are used, and gel times are on the order of seconds.
Thus, it would be desirable to provide for improved processing latitude in the preparation of PUU elastomers using polymers of propylene oxide. It would also be desirable to provide such improved processing latitude while maintaining fast demold times, or even decreasing same.
In addition, it is always desirable to improve the physical properties of PUU elastomers. In making cast elastomers, improvements in tensile and tear strength are desirable. In RIM elastomers, improvement in flexural modulus ratio, heat sag and impact strength are desired. Also, RIM elastomers often exhibit defects called "surface sinks" in which areas of the polymer are slightly depressed relative to the remaining surface. It is desired to decrease the amount of these surface sinks, particularly when a high quality surface is desired, such as for external automobile parts. It would also be desirable to provide an elastomer with improved thermal properties and in some instances improved moisture sensitivity.